1. Technical Field
The present invention relates to a position sensitive photoelectric sensor for sensing a distance to a target by a triangular range finding using a light, and a method of changing a reference distance set in the same.
2. Related Art
This type photoelectric sensor is called a position sensor or a displacement sensor in some cases, and used to measure a distance to a target (referred to as a “work” hereinafter) based on a triangular range finding using a light. As shown in FIG. 1, a light emitting device 102 and a light receiving device 103 are arrange at a predetermined interval in a sensor head 101. A light projected from the light emitting device 102 is passed through a projection lens 104 and then projected onto a work WK. Then, a light reflected from the work WK is passed through a reception lens 105 and incident on the light receiving device 103. As the light receiving device 103, PSD (position sensitive semiconductor device) or CCD (solid-state image sensor), which has a photosensitive surface in a predetermined range and is able to sense a spot position of the incident light on the photosensitive surface or a center-of-gravity position of a light quantity distribution, is employed.
In FIG. 1, when a position of the work WK comes closer to the sensor head 101 as shown by a broken line, the reflected light incident on the light receiving device 103 is changed as shown by a broken line. Therefore, the spot position of the received light on the photosensitive surface of the light receiving device 103 or the center-of-gravity position of the light quantity distribution is moved as indicated with an arrow. When the work WK is moved in the direction to go away from the sensor head 101, the spot position of the received light on the photosensitive surface of the light receiving device 103 or the center-of-gravity position of the light quantity distribution is moved in the opposite direction to that indicated with an arrow. Accordingly, a distance to the work WK or its displacement can be measured by sensing the spot position of the received light on the photosensitive surface of the light receiving device 103 or the center-of-gravity position of the light quantity distribution.
Normally such photoelectric sensor has functions of measuring a distance to the work WK and displaying it, and also displaying a compared result between a measured result and a reference distance (threshold value) as a binary signal and outputting it to the external device. Also, there is the photoelectric sensor that is specialized to have only a function of outputting the binary signal as the compared result between the distance to the work and the reference distance. Sometimes this sensor is called particularly a distance settable photoelectric switch. In such distance settable photoelectric switch, a split PD (photodiode) that is more inexpensive than above PSD or CCD can be used as the light receiving device.
As shown in FIG. 2, the split PD 103 as the light receiving device has the photosensitive surface divided into two parts, and individual received light quantity signals can be detected from respective split photosensitive surfaces. One split photosensitive surface is called an N-side (Near side) photosensitive surface, and the other split photosensitive surface is called an F-side (Far side) photosensitive surface. As shown in FIG. 2(b), when a received light spot SP is positioned on the boundary between the N-side photosensitive surface and the F-side photosensitive surface, equal received light quantity signals can be obtained from both split photosensitive surfaces. Also, as shown in FIG. 2(a), when the received light spot SP is shifted to the N-side photosensitive surface, a quantity of received light obtained from the N-side photosensitive surface becomes larger than a quantity of received light obtained from the F-side photosensitive surface (N>F). Conversely, as shown in FIG. 2(c), when the received light spot SP is shifted to the F side photosensitive surface, a quantity of received light obtained from the F-side photosensitive surface becomes larger than a quantity of received light obtained from the N-side photosensitive surface (N<F). Accordingly, if a situation shown in FIG. 2(b) is supposed as a reference distance, the binary signal indicating whether a distance to the work WK is near to the reference distance or far from the reference distance can be output based on that a difference in quantities of received lights obtained from the N-side photosensitive surface and the F-side photosensitive surface is positive or negative.
As can be understood from FIG. 1, in the above position sensitive photoelectric sensor, a measurable distance range is decided based on a size of the photosensitive surface of the light receiving device and constants of an optical system (an interval between the light emitting device 102 and the light receiving device 103, a magnification of the reception lens, and the like). When the position of the work is out of the measurable distance range, the received light spot of the reflected light is also out of the photosensitive surface of the light receiving device and thus the reflected light cannot be measured. When a magnification of the reception lens is increased, a wide distance range can be covered with the small light receiving device (photosensitive surface), but a resolving power is degraded. Therefore, the short-distance position sensitive photoelectric sensor and the long-distance position sensitive photoelectric sensor are provided in the related art.
Also, in the case of the distance settable photoelectric switch, since the reference distance applied to output the binary signal is decided based on a positional relationship between the light emitting device and the light receiving device, an angle, and the like, it is difficult for the user to change the reference distance arbitrarily. For example, in JP-A-6-168652 (hereinafter referred as Patent Literature 1), the distance settable photoelectric switch the reference distance of which the user can change by providing a mechanism, which changes/adjusts an angle of a light receiving axis, and the like, to the sensor head has been proposed.
However, in the distance settable photoelectric switch set forth in Patent Literature 1, since the reference distance is set only by the adjusting mechanism of the optical system, it is difficult to set the optimum reference distance. In particular, the setting varies widely among the workers (users). Also, in the position sensitive photoelectric sensor of the type that the distance to the work is measured, as described above, it is difficult to cover the wide measured distance range by one sensor.